The practice of acoustical well logging is well known in the oil and gas industry. One particular use of acoustic logging is to determine the points at which fluid or gas are entering or leaving the bore of a well. This information is particularly valuable when fluid or gas is entering or leaving the well through a leak in the casing. These casing leaks must often be plugged to assure proper well operations. Acoustic logging is also useful in determining whether or not there is channeling between different producing formations in a well.
One apparatus for locating leaks in casings and boreholes is disclosed in U.S. Pat. No. 2,210,417 issued Aug. 6, 1940 to Kinley. Kinley discloses a sound detector suspended from a cable so that it may be lowered into and withdrawn from a borehole. The detector comprises a sound detecting mechanism, such as a microphone, which responds to sound produced by the leaking liquid.
Another apparatus for determining the location of fluid entering or exiting a well is disclosed in U.S. Pat. No. 2,361,458 issued on Oct. 31, 1944 to Converse. This patent discloses an acoustic device having a sharp response to horizontal signals. Because of this response characteristic, the apparatus may be used to very precisely determine the points of entrance of fluid into a borehole. The apparatus is characterized as being sensitive to noises originating adjacent to the apparatus, while the effect of other noises in the same acoustical range, but vertically removed from the apparatus, is substantially eliminated.
Although devices for determining the location of a leak in a well such as those described above are well known, such devices are generally directionally insensitive. These devices identify only the depth at which the noise associated with the leak is being produced, not the direction from which the noise is coming. Information as to the direction in which a subsurface noise source is located can be very useful in oil and gas well operations.
One such use for directional noise information is in blowout relief well operations. One method of correcting a blowout is to drill relief wells to the vicinity of the well in the formation which is producing fluids or gas into a blowout. Determining the location of such production and drilling a relief well with the required accuracy are difficult tasks. One common technique relies on detecting aberrations in the propagation of magnetic waves in the earth. Such aberrations may be caused by the well casing of a blowout well. Another common technique detects resistivity differences between a blowout well casing and the earth. Such techniques can often be difficult to apply and in any event are of little use when a blowout occurs below the well casing or occurs in a well having no well casing.
Directional information is also useful in operations involving wells where channeling is occurring between different formations. Channeling involves the flow of fluids behind the well casing. Channeling may involve the leakage of fluid or gas from a producing to a non-producing formation, or the leakage of non-hydrocarbons into a producing formation. Such leakage may result in an eventual decrease in production from the well. One method of correcting such leaks is to perforate the well casing and force cement into the undesired channel, thus blocking the flow of fluid or gas through the channel. Although the depth at which the perforation should be made may be determined using conventional noise logging techniques, the proper circumferential location in which to perforate the casing is difficult to determine.
There continues to be a need in the oil and gas industry for improved apparatus and methods for determining the direction of a subsurface noise source.